Preparation method of lithium manganese iron phosphate positive electrode material and application thereof

Abstract

The application belongs to the technical field of lithium ion battery materials, and particularly relates to a preparation method of a lithium manganese iron phosphate positive electrode material and application thereof. The preparation method of the lithium manganese iron phosphate positive electrode material comprises the following steps: mixing lithium manganese phosphate material, an iron source, a phosphorus source, a lithium source, a dopant and a carbon source, sand milling, spray drying to obtain a precursor, and subjecting the precursor to one-stage sintering, cooling and two-stage sintering to obtain the lithium manganese iron phosphate positive electrode material. The precursor is subjected to two-stage sintering, one-stage sintering promotes diffusion of manganese and iron ions at high temperature, in-situ solid solution of manganese and iron is realized, two-stage sintering is carried out in an environment lower than the temperature of one-stage sintering after cooling, and structural defects generated in the material during high-temperature solid solution are eliminated, the prepared lithium manganese iron phosphate material is rich in iron on the outer layer, not only effectively reducing dissolution of manganese ions, but also improving the electrical conductivity of the material, and thus the positive electrode material has excellent electrochemical performance and long cycle performance.

Description

Technical Field

[0001] This invention belongs to the field of lithium-ion battery material technology, specifically relating to a method for preparing lithium manganese iron phosphate cathode material and its application. Background Technology

[0002] In recent years, the use of green, renewable, and carbon-free energy has become an inevitable trend and a necessary choice for pursuing sustainable development. Lithium-ion batteries are important electrochemical energy conversion and storage devices, and due to their advantages such as high energy density, portability, and low self-discharge, they have become one of the most important new energy realization methods. Currently, lithium iron phosphate (LFP) is the most commonly used commercial lithium-ion battery cathode material, attributed to its good platform stability, low cost, high safety, and good thermal stability, thus it is widely used in electric vehicles and energy storage. However, compared to other cathode materials, the theoretical energy density of lithium iron phosphate (LiFePO4, LFP) is relatively low. To further improve the energy density of lithium-ion batteries, it is necessary to develop high-energy-density LFP cathode materials.

[0003] Lithium manganese iron phosphate (LiMn) 1-x Fe x Lithium iron phosphate (LiPO4, LMP) can be considered a solid solution of lithium iron phosphate and lithium manganese phosphate (LiMnPO4, LMP). It has the same olivine structure as lithium iron phosphate and therefore possesses similar good safety. However, it has a higher redox potential (MnPO4). 2+ / Mn 3+ ~4.1V vs. Fe 2+ / Fe 3+ With a voltage of ~3.4V and a theoretical specific capacity comparable to lithium iron phosphate, lithium manganese iron phosphate cathode material exhibits higher energy density, making it an important candidate material to replace lithium iron phosphate. However, on the one hand, Mn... 3+ The Jahn-Teller effect exists, leading to structural distortion and deteriorated electrochemical performance during cycling; on the other hand, Mn at high temperatures... 3+ Significant dissolution can easily lead to manganese loss and structural distortion, resulting in poor cycling performance of lithium manganese iron phosphate under high manganese conditions. To improve the cycling properties of lithium manganese iron phosphate, current mainstream methods include doping and coating (such as carbon coating, oxide coating, and Li₂MF₆ coating), but most of these methods suffer from complex preparation processes and limited performance improvements.

[0004] Therefore, this invention is proposed. Summary of the Invention

[0005] In view of the problem of poor electrical and cycling performance of lithium manganese iron phosphate cathode materials due to manganese ion loss in the prior art, the purpose of this invention is to provide a method for preparing lithium manganese iron phosphate cathode materials and their application, aiming to reduce manganese dissolution during the cycling process of lithium manganese iron phosphate materials and improve their electrochemical and cycling performance.

[0006] To achieve this objective, the present invention adopts the following technical solution:

[0007] In a first aspect, the present invention provides a method for preparing a lithium manganese iron phosphate cathode material, the method comprising the following steps:

[0008] The precursor is obtained by mixing lithium manganese phosphate material, iron source, second phosphorus source, second lithium source, dopant M and carbon source in a second process and drying in a second process; the precursor is then subjected to a first-stage sintering, cooling and a second-stage sintering to obtain lithium manganese iron phosphate cathode material.

[0009] In an optional embodiment, the atmosphere for the first-stage sintering, cooling, and second-stage sintering processes is nitrogen or argon, with a gas flow rate of 10-100 cm⁻¹. 3 / min, preferably 20 cm 3 / min.

[0010] In an optional implementation, the sintering temperature of the first sintering stage is 700~850℃, and the sintering time is 6-12h.

[0011] In an optional implementation, the cooling refers to cooling to 200-350°C after a sintering stage.

[0012] In an optional implementation, the sintering temperature of the two-stage sintering is 600-750℃, and the sintering time is 4-8h.

[0013] In an optional embodiment, the second mixing includes adding the lithium manganese phosphate material, the iron source, the second phosphorus source, the second lithium source, the dopant M, and the carbon source to water to prepare a second mixture, wherein the total mass fraction of the lithium manganese phosphate material, the iron source, the second phosphorus source, the second lithium source, the dopant M, and the carbon source in the second mixture is 20%-60%; and subjecting the second mixture to a second sand milling to control the particle size D50 of the solids in the second mixture to be 0.35~0.60 µm.

[0014] In an optional embodiment, the second drying includes spray drying, wherein the inlet air temperature during the drying process is 180~240°C and the outlet air temperature is 80~120°C.

[0015] In an optional embodiment, the iron source is a soluble iron salt or an iron hydroxide.

[0016] In an optional embodiment, the dopant M comprises at least one of a compound of magnesium, titanium, vanadium, niobium, chromium, and cerium.

[0017] In an optional embodiment, the carbon source includes at least one selected from glucose, sucrose, polyvinyl alcohol, polyacrylamide, polyvinylpyrrolidone, chitosan, melamine, fructose, cyclodextrin, and polyethylene glycol.

[0018] In an optional implementation, the molar ratio of lithium in the second lithium source, iron in the iron source, and phosphorus in the second phosphorus source is (1-1.05):1:(1-1.03).

[0019] In an optional implementation, the molar ratio of iron in the iron source to manganese in the lithium manganese phosphate material is (1-5):(5-9).

[0020] In an optional embodiment, the molar ratio of the metal element in the dopant M to the iron element in the iron source is (0.005-0.05):1.

[0021] In an optional implementation, the mass of the carbon source is 10%-20% of the theoretical yield mass of carbon-free lithium manganese iron phosphate.

[0022] In an optional implementation, the preparation method of the lithium manganese phosphate material includes: mixing a manganese source, a first phosphorus source, a first lithium source and a dopant N in a first mixing process, followed by freeze drying to obtain a dried material; and sintering the dried material to obtain the lithium manganese phosphate material.

[0023] In an optional embodiment, the manganese source includes at least one of manganese oxides, manganese hydroxides, soluble manganese salts, manganese carbonate, and manganese phosphate.

[0024] In an optional embodiment, the first or second phosphorus source includes at least one of phosphoric acid, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, and phosphorus pentoxide.

[0025] In an optional embodiment, the first lithium source or the second lithium source includes at least one of lithium carbonate, lithium hydroxide, lithium phosphate, lithium oxalate, lithium acetate, and lithium nitrate.

[0026] In an optional embodiment, the dopant N includes at least one of a compound of magnesium, titanium, nickel, aluminum, vanadium, niobium, cobalt, chromium, and cerium.

[0027] In an optional implementation, the first mixing includes adding the manganese source, the first phosphorus source, the first lithium source and the dopant N to water to prepare a first mixture, wherein the total mass fraction of the manganese source, the first phosphorus source, the first lithium source and the dopant N in the first mixture is 20%-60%; and the first mixture is subjected to a first sand milling to control the particle size D50 of the solids in the first mixture to be 0.2~0.4 µm.

[0028] In an optional implementation, the molar ratio of manganese in the manganese source, lithium in the first lithium source, and phosphorus in the first phosphorus source is 1:(1-1.05):(1-1.03).

[0029] In an optional embodiment, the molar ratio of the metal element in the dopant N to the manganese element in the manganese source is (0.005-0.05):1.

[0030] In an optional embodiment, the sintering atmosphere includes at least one of nitrogen (N2), argon (Ar), 95% nitrogen / 5% hydrogen (95% N2 / 5% H2), and 95% argon / 5% hydrogen (95% Ar / 5% H2) (here, the gas percentage is a volume percentage), and the gas flow rate is 10-100 cm⁻¹. 3 / min, preferably 20 cm 3 / min.

[0031] In an optional embodiment, the sintering temperature is 500-650℃ and the sintering time is 2-8h.

[0032] Secondly, the present invention also provides the application of lithium iron phosphate cathode material prepared by the method for preparing lithium manganese iron phosphate cathode material as described in the first aspect in lithium-ion batteries.

[0033] Compared with the prior art, the present invention has the following beneficial effects:

[0034] (1) The surface of the lithium manganese iron phosphate cathode material particles provided by the present invention is an iron-rich layer. On the one hand, it can effectively reduce the dissolution of manganese ions and improve the cycle performance of the cathode material. On the other hand, the iron-rich particle surface is conducive to improving the conductivity of the cathode material, thereby reducing the amount of carbon coating, increasing the compaction density of the lithium manganese iron phosphate material, and thus enabling the cathode material to have better electrochemical performance and cycle performance.

[0035] (2) This invention involves a first-stage sintering, cooling, and second-stage sintering process on the precursor. The first-stage sintering at high temperature promotes the in-situ diffusion of manganese and iron ions to achieve solid solution. After cooling, the second-stage sintering is performed at a lower temperature than the first-stage sintering temperature, eliminating structural defects and residual stress generated inside the material during the high-temperature solid solution process. This ultimately yields a lithium manganese iron phosphate cathode material with iron-rich particle surfaces and uniform elemental distribution in the bulk phase. Moreover, the synthesis method of this invention for preparing lithium manganese iron phosphate cathode material is simple, the process is controllable, and it is easy to achieve large-scale production. Attached Figure Description

[0036] The accompanying drawings are provided to further understand the technical solutions herein and form part of the specification. They are used together with the embodiments of this application to explain the technical solutions herein and do not constitute a limitation on the technical solutions herein.

[0037] Figure 1 This is a schematic diagram of the synthesis process of lithium manganese iron phosphate cathode material in this invention;

[0038] Figure 2 SEM images of primary particles of lithium iron manganese phosphate cathode material prepared in Examples 1(a), 5(b), and 6(c) of this invention;

[0039] Figure 3 The image shows the XRD pattern of the lithium manganese iron phosphate cathode material prepared in Example 5 of this invention. Detailed Implementation

[0040] The technical solution of the present invention will be further illustrated below through specific embodiments. Those skilled in the art should understand that the embodiments described are merely illustrative of this disclosure and should not be construed as limiting the scope of the invention.

[0041] It should be noted that, unless otherwise specified in the examples, the conditions should be performed under standard conditions or conditions recommended by the manufacturer. Reagents or instruments whose manufacturers are not specified are all commercially available products.

[0042] This invention provides a method for preparing lithium manganese iron phosphate cathode material, the method comprising the following steps:

[0043] The precursor is obtained by mixing lithium manganese phosphate material, iron source, second phosphorus source, second lithium source, dopant M and carbon source in a second process and drying in a second process; the precursor is then subjected to a first-stage sintering, cooling and a second-stage sintering to obtain lithium manganese iron phosphate cathode material.

[0044] This invention involves a first-stage sintering, cooling, and second-stage sintering process on the precursor. The first-stage sintering at high temperature promotes the diffusion of manganese and iron ions, achieving in-situ solid solution of manganese and iron. The second-stage sintering after cooling is similar to tempering. After cooling, the second-stage sintering is performed at a lower temperature (compared to the first-stage sintering temperature). On the one hand, this can eliminate structural defects and residual stress generated inside the material during the high-temperature solid solution process and improve the diffusion of lithium ions. On the other hand, it can further promote the diffusion of manganese and iron elements, thereby obtaining a lithium manganese iron phosphate cathode material with iron-rich particle surface and uniform element distribution in the bulk phase.

[0045] In some embodiments, the second mixing includes adding lithium manganese phosphate material, an iron source, a second phosphorus source, a second lithium source, dopant M, and a carbon source to water to prepare a second mixture.

[0046] In some embodiments, the second mixing includes adding lithium manganese phosphate material, iron source, second phosphorus source, second lithium source, dopant M and carbon source to water to prepare a second mixture, and then subjecting the second mixture to a second sand milling to ensure that the solid particle size in the second mixture meets the standard.

[0047] In some embodiments, the second drying is carried out by spray drying, wherein the inlet air temperature of the spray drying is 180~240℃, for example, 180℃, 200℃, 220℃ or 240℃, and the outlet air temperature is 80~120℃, for example, 80℃, 100℃, 120℃ or 120℃, but not limited to the listed values, and other unlisted values ​​within the above range are also applicable.

[0048] In some embodiments, the total mass fraction of lithium manganese phosphate material, iron source, second phosphorus source, second lithium source, dopant M and carbon source in the second mixture is 20%-60%, for example, it can be 20%, 30%, 40% or 60%, but is not limited to the listed values, and other unlisted values ​​within the above range are also applicable.

[0049] In some embodiments, the particle size D50 of the solids in the second mixture is 0.35 to 0.60 µm, for example, it can be 0.35 µm, 0.40 µm, 0.45 µm, 0.50 µm or 0.60 µm, but is not limited to the listed values, other unlisted values ​​within the above range are also applicable.

[0050] In the process of preparing lithium manganese iron phosphate material, the particle size of the solid in the second mixture affects the electrochemical properties and compaction density of the obtained lithium manganese iron phosphate cathode material. If the solid particle size is too large or too small, the diffusion rate of lithium ions will decrease, resulting in a lower specific capacity of the lithium battery prepared by the cathode material.

[0051] In some embodiments, the molar ratio of lithium in the second lithium source, iron in the iron source, and phosphorus in the second phosphorus source is (1-1.05):1:(1-1.03), for example, it can be 1:1:1, 1.02:1:1.01, 1.04:1:1.02, or 1.05:1:1.03, but it is not limited to the listed values. Other unlisted values ​​within the above range are also applicable.

[0052] In some embodiments, the molar ratio of iron in the iron source to manganese in the lithium manganese phosphate material is (1-5):(5-9), for example, it can be 5:5, 4:6, 3:7, 2:8 or 1:9, but is not limited to the listed values. Other unlisted values ​​within the above range are also applicable.

[0053] In some embodiments, the molar ratio of the metal element in the dopant M to the iron element in the iron source is (0.005-0.05):1, for example, it can be 0.005:1, 0.015:1, 0.03:1, 0.04:1 or 0.05:1, but is not limited to the listed values. Other unlisted values ​​within the above range are also applicable.

[0054] In some embodiments, the mass of the carbon source is 10%-20% of the theoretical yield of carbon-free lithium manganese iron phosphate, for example, it can be 10%, 15% or 20%, but is not limited to the listed values, and other unlisted values ​​within the above range are also applicable.

[0055] In some embodiments, the sintering temperature of the sintering section is 700~850℃, for example, 700℃, 750℃, 800℃ or 850℃, and the sintering time is 6-12h, for example, 6h, 8h, 10h or 12h, but not limited to the listed values, other unlisted values ​​within the above range are also applicable.

[0056] In some embodiments, the cooling refers to cooling to 200-350°C after a sintering stage, for example, 200°C, 250°C, 300°C or 350°C, but not limited to the listed values. Other unlisted values ​​within the above range are also applicable.

[0057] In some embodiments, the sintering temperature of the two-stage sintering is 600-750℃, for example, 600℃, 650℃, 700℃ or 750℃, and the sintering time is 4-8h, for example, 4h, 5h, 7h or 8h, but not limited to the listed values. Other unlisted values ​​within the above range are also applicable.

[0058] In the preparation of lithium manganese iron phosphate materials, this invention employs a higher first-stage sintering temperature than the second-stage sintering temperature to promote the diffusion of manganese to the particle surface and iron to the particle interior, thereby achieving in-situ solid solution of manganese and iron. Furthermore, the second-stage sintering temperature further affects the degree of iron-manganese solid solution. If the sintering temperature is too high, manganese will continue to diffuse outwards, resulting in a higher manganese content on the particle surface; if the sintering temperature is too low, atomic diffusion is poor, leading to insufficient defect elimination.

[0059] In some embodiments, the preparation method of the lithium manganese phosphate material includes: mixing a manganese source, a first phosphorus source, a first lithium source and a dopant N in a first mixing process, freeze-drying to obtain a dried material; and sintering the dried material to obtain the lithium manganese phosphate material.

[0060] In some embodiments, the first mixing includes adding a manganese source, a first phosphorus source, a first lithium source, and a dopant N to water to prepare a first mixture.

[0061] In some embodiments, the first mixing includes adding a manganese source, a first phosphorus source, a first lithium source and dopant N to water to prepare a first mixture, and then subjecting the first mixture to a first sand milling to ensure that the solid particle size in the first mixture meets the standard.

[0062] In some embodiments, the freeze-drying refers to freezing the mixture after the first milling with liquid nitrogen and then subjecting it to sublimation drying in a vacuum environment.

[0063] In the method for preparing lithium manganese phosphate materials, the present invention employs a freeze-drying step to prevent excessive agglomeration of lithium manganese phosphate particles in the first stage.

[0064] In some embodiments, the total mass fraction of manganese source, first phosphorus source, first lithium source and dopant N in the first mixture is 20%-60%, for example, it can be 20%, 40% or 60%, but is not limited to the listed values. Other unlisted values ​​within the above range are also applicable.

[0065] In some embodiments, the particle size D50 of the solids in the first mixture is 0.2 to 0.4 µm, for example, it can be 0.2 µm, 0.25 µm, 0.3 µm, 0.35 µm or 0.4 µm, but is not limited to the listed values, other unlisted values ​​within the above range are also applicable.

[0066] In some embodiments, the molar ratio of manganese in the manganese source, lithium in the first lithium source, and phosphorus in the first phosphorus source is 1:(1-1.05):(1-1.03), for example, it can be 1:1:1, 1:1.03:1.02, 1:1.04:1.02, or 1:1.05:1.03, but it is not limited to the listed values. Other unlisted values ​​within the above range are also applicable.

[0067] In some embodiments, the molar ratio of the metal element in the dopant N to the manganese element in the manganese source is (0.005-0.05):1, for example, it can be 0.005:1, 0.015:1, 0.03:1, 0.04:1 or 0.05:1, but is not limited to the listed values. Other unlisted values ​​within the above range are also applicable.

[0068] In some embodiments, the sintering temperature is 500-650°C, for example, 500°C, 550°C, 600°C or 650°C, and the sintering time is 2-8h, for example, 2h, 4h, 6h or 8h, but not limited to the listed values. Other unlisted values ​​within the above range are also applicable.

[0069] The lithium iron phosphate cathode material of this invention is shown in the accompanying drawings. Figure 1 The preparation is carried out according to the synthesis process shown.

[0070] Example 1

[0071] This embodiment provides a method for preparing lithium manganese iron phosphate cathode material, the method comprising the following steps:

[0072] S1. Weigh lithium carbonate, manganese tetroxide, phosphoric acid, and nickel oxide according to a lithium:manganese:phosphorus:nickel molar ratio of 1.04:1:1.02:0.015 and add them to deionized water to prepare a mixture with a total mass fraction of 60%. The mixture is then milled until the particle size D50 of the solids in the mixture is 0.25 µm. It is then frozen with liquid nitrogen and subsequently sublimated (freeze-dried) in a vacuum oven to obtain a dried material. The freeze-dried powder is then sintered in a 95% N2 / 5% H2 atmosphere with a gas flow rate of 20 cm⁻¹. 3 The sintering rate was 620℃, the holding time was 8h, and lithium manganese phosphate material was obtained.

[0073] S2. First, disperse lithium manganese phosphate powder in deionized water. Weigh lithium acetate, ferrous sulfate, phosphoric acid, and niobium dioxide (with a manganese:iron molar ratio of 7:3) according to a lithium:iron:phosphorus:niobium molar ratio of 1.04:1:1.02:0.015 and add them to the dispersion. Then, weigh glucose and fructose (with a mass ratio of 1:1, and a total mass of 15% of the theoretical yield of carbon-free lithium manganese iron phosphate) and add them to the dispersion to prepare a mixture with a total mass fraction of 40%. Mill the mixture until the particle size D50 of the solids in the mixture is 0.40. At a particle size of µm, spray drying was performed with an inlet air temperature of 200℃ and an outlet air temperature of 90℃ to obtain a precursor dried material. The precursor dried material was then sintered under a N2 atmosphere. The first sintering stage temperature was 800℃, held for 8 hours, then cooled to 200℃, and then heated to the second sintering stage at 680℃ for 4 hours. The airflow velocity during sintering was 20 cm⁻¹. 3 / min. After discharge, the material is crushed and then passed through a 300-mesh sieve to obtain lithium manganese iron phosphate cathode material (LM). 0.7 F 0.3 P).

[0074] Example 2

[0075] This embodiment provides a method for preparing lithium manganese iron phosphate cathode material, the method comprising the following steps:

[0076] S1. Weigh lithium carbonate, manganese tetroxide, phosphoric acid, and nickel oxide according to a lithium:manganese:phosphorus:nickel molar ratio of 1:1:1:0.005 and add them to deionized water to prepare a mixture with a total mass fraction of 20%. The mixture is then milled until the particle size D50 of the solids in the mixture is 0.20 µm. It is then frozen with liquid nitrogen and subsequently sublimated (freeze-dried) in a vacuum oven to obtain a dried material. The freeze-dried powder is then sintered in a 95% N2 / 5% H2 atmosphere with a gas flow rate of 20 cm⁻¹. 3 The sintering rate was 500℃, the holding time was 6h, and lithium manganese phosphate material was obtained.

[0077] S2. First, disperse lithium manganese phosphate powder in deionized water. Weigh lithium acetate, ferrous sulfate, phosphoric acid, and niobium dioxide (with a manganese:iron molar ratio of 5:5) according to a lithium:iron:phosphorus:niobium molar ratio of 1:1:1:0.005 and add them to the dispersion. Then, weigh glucose and fructose (with a mass ratio of 1:1, and a total mass of 10% of the theoretical yield of carbon-free lithium manganese iron phosphate) and add them to the dispersion to prepare a mixture with a total mass fraction of 40%. Mill the mixture until the particle size D50 of the solids in the mixture is 0.35. At a particle size of µm, spray drying was performed with an inlet air temperature of 240℃ and an outlet air temperature of 80℃ to obtain a precursor dried material. The precursor dried material was then sintered under a N2 atmosphere. The first sintering stage temperature was 700℃ for 12 hours, followed by a decrease to 350℃, and then a second sintering stage at 600℃ for 8 hours. The airflow velocity during sintering was 20 cm⁻¹. 3 / min. After discharge, the material is crushed and then passed through a 300-mesh sieve to obtain lithium manganese iron phosphate cathode material (LM). 0.5 F 0.5 P).

[0078] Example 3

[0079] This embodiment provides a method for preparing lithium manganese iron phosphate cathode material, the method comprising the following steps:

[0080] S1. Weigh lithium carbonate, manganese tetroxide, phosphoric acid, and nickel oxide according to the molar ratio of lithium:manganese:phosphorus:nickel of 1.05:1:1.03:0.05 and add them to deionized water to prepare a mixture with a total mass fraction of 50%. The mixture is then milled until the particle size D50 of the solid in the mixture is 0.40 µm. It is then frozen with liquid nitrogen and then sublimated (freeze-dried) in a vacuum oven to obtain a dried material. Subsequently, the freeze-dried powder is sintered in a 95% N2 / 5% H2 atmosphere with a gas flow rate of 20 cm3 / min, a sintering temperature of 650℃, and a holding time of 2h to obtain lithium manganese phosphate material.

[0081] S2. First, disperse lithium manganese phosphate powder in deionized water. Weigh lithium acetate, ferrous sulfate, phosphoric acid, and niobium dioxide (with a manganese:iron molar ratio of 9:1) according to a lithium:iron:phosphorus:niobium molar ratio of 1.05:1:1.03:0.05 and add them to the dispersion. Then, weigh glucose and fructose (with a mass ratio of 1:1, and a total mass of 20% of the theoretical yield of carbon-free lithium manganese iron phosphate) and add them to the dispersion to prepare a mixture with a total mass fraction of 50%. Mill the mixture until the particle size D50 of the solids in the mixture is 0.60. At a particle size of µm, spray drying was performed with an inlet air temperature of 180℃ and an outlet air temperature of 120℃ to obtain a precursor dried material. The precursor dried material was then sintered under a N2 atmosphere. The first sintering stage temperature was 850℃, held for 6 hours, then cooled to 250℃, and then heated to the second sintering stage at 750℃ for 6 hours. The airflow velocity during sintering was 20 cm⁻¹. 3 / min. After discharge, the material is crushed and then passed through a 300-mesh sieve to obtain lithium manganese iron phosphate cathode material (LM). 0.9 F 0.1 P).

[0082] Example 4

[0083] S1. Weigh lithium carbonate, manganese tetroxide, phosphoric acid, and nickel oxide according to a lithium:manganese:phosphorus:nickel molar ratio of 1.05:1:1.02:0.01, and add them to deionized water to prepare a mixture with a total mass percentage of 40%. The mixture is then milled until the particle size D50 of the solids in the mixture is 0.25 µm. It is then frozen with liquid nitrogen and subsequently sublimated (freeze-dried) in a vacuum oven to obtain a dried material. The freeze-dried powder is then sintered in a 95% N2 / 5% H2 atmosphere with a gas flow rate of 20 cm⁻¹. 3 The sintering rate was 600℃, the holding time was 8h, and lithium manganese phosphate material was obtained.

[0084] S2. First, disperse lithium manganese phosphate powder in deionized water. Weigh lithium acetate, ferrous sulfate, phosphoric acid, and niobium dioxide (with a manganese:iron molar ratio of 6:4) according to a lithium:iron:phosphorus:niobium molar ratio of 1.05:1:1.02:0.01 and add them to the dispersion. Then, weigh glucose and polyethylene glycol (with a mass ratio of 1:1, and a total mass of 15% of the theoretical yield of carbon-free lithium manganese iron phosphate) and add them to the dispersion to prepare a mixture with a total mass fraction of 30%. Mill the mixture until the particle size D50 of the solids in the mixture is 0.40. At a particle size of µm, spray drying was performed with an inlet air temperature of 200℃ and an outlet air temperature of 90℃ to obtain a precursor dried material. The precursor dried material was then sintered under a N2 atmosphere. The first sintering stage temperature was 800℃, held for 8 hours, then cooled to 200℃, and then heated to the second sintering stage at 680℃ for 4 hours. The airflow velocity during sintering was 20 cm⁻¹. 3 / min. After discharge, the material is crushed and then passed through a 300-mesh sieve to obtain lithium manganese iron phosphate cathode material (LM). 0.6 F 0.4 P).

[0085] Example 5

[0086] S1. Compared with Example 1, the molar ratio of lithium:manganese:phosphorus:nickel was changed to 1.05:1:1.02:0.015, while the other steps remained unchanged.

[0087] S2. Compared with Example 1, the molar ratio of lithium:iron:phosphorus:niobium was changed to 1.05:1:1.02:0.01, the carbon source was changed to glucose and polyethylene glycol, the cooling temperature was changed to 250°C, and the temperature of the second-stage sintering stage was changed to 700°C. The remaining steps remained unchanged.

[0088] Example 6

[0089] S1. Compared with Example 4, the total mass fraction of the mixture was changed to 30%, and the particle size D50 of the sand milling was changed to 0.3µm. The other steps remained unchanged.

[0090] S2. Compared with Example 4, the particle size D50 of the sand mill was changed to 0.45µm, the molar ratio of manganese to iron was changed to 7:3, the carbon source was changed to glucose and polyvinyl alcohol, and the total mass of the two was 12% of the theoretical output mass of lithium manganese iron phosphate; the inlet air temperature of the spray drying was changed to 180°C and the outlet air temperature was changed to 80°C, and the other steps remained unchanged.

[0091] Example 7

[0092] S1. Compared with Example 4, the sintering temperature was changed to 650°C, while the other steps remained unchanged.

[0093] S2. Compared with Example 4, the carbon source was changed to glucose and cyclodextrin, the temperature of the second sintering stage was changed to 720°C, and the other steps remained unchanged.

[0094] Example 8

[0095] S1. Compared with Example 4, the particle size D50 of the solid in the mixture was changed to 0.30µm after sand milling, and the molar ratio of manganese to iron was changed to 8:2. The other steps remained unchanged.

[0096] S2, the same as the steps in Example 4.

[0097] Example 9

[0098] Compared with Example 1, the particle size D50 of the solid in the mixture during sand milling in step S2 was changed to 0.30 µm, while the other steps remained unchanged.

[0099] Example 10

[0100] Compared with Example 1, the particle size D50 of the solid in the mixture during sand milling in step S2 was changed to 0.50 µm, while the other steps remained unchanged.

[0101] Example 11

[0102] Compared with Example 1, the molar ratio of manganese to iron in step S2 was changed to 5:5, while the other steps remained unchanged.

[0103] Example 12

[0104] Compared with Example 1, the molar ratio of manganese to iron in step S2 was changed to 9:1, while the other steps remained unchanged.

[0105] Example 13

[0106] Compared with Example 1, the temperature of the second-stage sintering phase in step S2 was changed to 600°C, while the other steps remained unchanged.

[0107] Example 14

[0108] Compared with Example 1, the temperature of the second-stage sintering phase in step S2 was changed to 750°C, while the other steps remained unchanged.

[0109] Comparative Example 1

[0110] Compared with Example 4, the iron source was added in step S1 to prepare lithium manganese iron phosphate cathode material.

[0111] S1. Change the molar ratio of lithium:iron:manganese:phosphorus:nickel:niobium to 1.05:1:1:1.02:0.01:0.01, and leave the other steps unchanged.

[0112] S2. The lithium manganese iron phosphate material obtained in step S1 is mixed with a carbon source (glucose and polyethylene glycol) and subjected to a two-stage sintering process. The sintering process is the same as step S2 in Example 4.

[0113] Comparative Example 2

[0114] Compared with Example 5, the second sintering process is not performed in step S2, and the other steps remain unchanged.

[0115] Comparative Example 3

[0116] Compared to Example 5, in step S2, the soluble ferrous sulfate was replaced with the insoluble ferric phosphate, while the other steps remained unchanged.

[0117] The lithium manganese iron phosphate cathode materials prepared in Examples 1-14 and Comparative Examples 1-3 were subjected to specific surface area and compaction density tests, and their electrochemical performance was evaluated by configuring them into coin cells. The specific steps included:

[0118] (1) Specific surface area test

[0119] The specific surface area of ​​cathode material powder particles was determined using a Micro ASAP2460 fully automated specific surface area analyzer.

[0120] (2) Compacted density test

[0121] The compaction density of the cathode material powder was determined using an FTYS-50KN powder compaction density meter.

[0122] (3) Preparation of button cells

[0123] The lithium manganese iron phosphate cathode material, conductive agent acetylene black, and adhesive polyvinylidene fluoride were uniformly mixed in N-methylpyrrolidone at a mass ratio of 92:4:4 to form a slurry. The slurry was then coated onto aluminum foil and dried in a vacuum drying oven. The cathode was then pressed into a positive electrode sheet using a tablet press, while the negative electrode sheet was a lithium metal sheet. The electrolyte was a 1 mol / L lithium hexafluorophosphate-ethylene carbonate:dimethyl carbonate (LiPF6-EC:DMC, volume ratio 1:1). A polypropylene porous membrane was used as the separator. The battery assembly was carried out in an argon glove box.

[0124] (4) Electrochemical performance testing

[0125] The assembled coin cells were subjected to electrochemical performance tests within a charge / discharge voltage range of 2.5–4.5 V. The test results are shown in Table 1.

[0126] Table 1

[0127]

[0128] From the attached diagram, we can conclude that:

[0129] Figure 2 The SEM images show that the primary particle size of the lithium manganese iron phosphate material prepared by this invention is 300-500 nm, and the particle size is uniform. Figure 3 The XRD pattern shows that the lithium manganese iron phosphate solid solution material prepared by the present invention is a pure phase, and no two-phase separation or impurity phases are observed.

[0130] As can be seen from Table 1:

[0131] As can be seen from Examples 1-14, the lithium manganese iron phosphate cathode material prepared by the present invention through first constructing a lithium iron phosphate precursor slurry coated with lithium manganese phosphate precursor (LMP@LFP), and then through a first-stage sintering, cooling and second-stage sintering process to prepare in-situ solid solution has high specific capacity, long cycle performance and high compaction density.

[0132] A comparison of Examples 1 and 9-10 shows that, in the preparation process of the lithium manganese iron phosphate cathode material of the present invention, the particle size of the solid in the mixture after sand milling in step S2 affects its performance. When the solid particle size is controlled within a suitable range, the electrical performance of the obtained cathode material is better; however, when the solid particle size is too large or too small, the diffusion rate of lithium ions in the material will decrease, the number of surface reaction active sites will decrease, and thus the specific capacity of the lithium battery prepared by the cathode material will decrease.

[0133] A comparison of Examples 1 and 11-12 shows that the molar ratio of manganese to iron in the preparation process of the lithium manganese iron phosphate cathode material of the present invention affects its performance. If the manganese-iron ratio is low, the manganese leaching problem is small, resulting in an increase in the specific capacity of the lithium battery made from it, an increase in the powder compaction density of the material, and better cycle performance. If the manganese-iron ratio is high, the manganese leaching problem is aggravated, leading to a decrease in the specific capacity of the lithium battery made from it, a decrease in the powder compaction density of the material, and a deterioration in cycle performance.

[0134] A comparison of Examples 1 and 13-14 shows that the performance of the lithium manganese iron phosphate cathode material prepared by the present invention is affected by the temperature of the two-stage sintering. If the sintering temperature is low, the internal defect elimination effect is good, and the specific capacity of the cathode material is higher and the cycle performance is better. If the sintering temperature is too high, the manganese content on the particle surface increases, the internal defect elimination effect is poor, and the specific capacity of the cathode material is lower and the cycle performance deteriorates.

[0135] Comparing Example 4 and Comparative Example 1, it can be seen that in Comparative Example 1, the precursor of lithium manganese iron phosphate was formed during the initial mixing in step S1, and then lithium manganese iron phosphate was obtained through sintering, without the process of coating lithium manganese phosphate with lithium iron phosphate precursor slurry and then dissolving it in situ. From the data results, Comparative Example 1 is worse than Example 4 in both specific capacity and compaction density, especially in long-cycle performance. This is because the LMP@LFP precursor structure and preparation process constructed in Example 4 can suppress Mn... 3+ The dissolution during cycling results in better cycling performance of the cathode material. In contrast, the method in Comparative Example 1, which directly mixes and sintersects lithium, iron, manganese, and phosphorus sources to prepare lithium manganese iron phosphate, leads to smaller particle size and larger specific surface area during synthesis, while also resulting in poorer conductivity and capacity.

[0136] Comparing Example 5 and Comparative Example 2, it can be seen that Comparative Example 2 did not perform a second-stage sintering in step S2, thus lacking a low-temperature tempering process. The data shows that the specific capacity and cycle performance of the cathode material in Comparative Example 2 are significantly worse than those in Example 5. This is because the two-stage sintering process in step S2 of Example 5 has distinctly different functions. The first stage of sintering is a high-temperature holding stage, which utilizes the rapid diffusion rate of manganese atoms at high temperatures to promote the in-situ solid solution of manganese and iron, thereby preparing lithium manganese iron phosphate. The second stage of sintering, on the other hand, eliminates structural defects and residual stresses generated within the material during the high-temperature solid solution process at low temperatures, thereby improving lithium-ion diffusion, ensuring the perfect utilization of the cathode material's capacity, and reducing degradation during long-cycle cycling.

[0137] Comparing Example 5 and Comparative Example 3, it can be seen that the iron source used in the secondary mixing process of Comparative Example 3 is insoluble iron phosphate, while Example 5 uses soluble iron source ferrous sulfate. The data shows that the specific capacity, cycle performance, and compaction density of the cathode material in Comparative Example 3 are significantly worse than those in Example 5. This is because the coating effect of insoluble iron salts on the surface of lithium manganese phosphate is poor, and the in-situ solid solution effect is also inferior to that of soluble iron salts. Therefore, the electrochemical performance is not as good as that of Example 5. The low compaction density is due to the poor coating effect of insoluble iron salts, resulting in an excessively high proportion of lithium manganese phosphate, which reduces the overall compaction density of the material.

[0138] The applicant declares that the above description is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention fall within the protection and disclosure scope of the present invention.

Claims

1. A method for preparing a lithium manganese iron phosphate cathode material, characterized in that, Includes the following steps: A precursor is obtained by mixing lithium manganese phosphate material, an iron source, a second phosphorus source, a second lithium source, dopant M, and a carbon source in a second process and then drying. The precursor is then subjected to a first-stage sintering, a cooling process, and a second-stage sintering to obtain lithium iron manganese phosphate cathode material. The first-stage sintering temperature is 700-850℃, and the sintering time is 6-12h. The cooling process involves cooling to 200-350℃ after the first-stage sintering. The second-stage sintering temperature is 600-750℃, and the sintering time is 4-8h. The iron source is a soluble iron salt or an iron hydroxide.

2. The method for preparing lithium manganese iron phosphate cathode material according to claim 1, characterized in that, The atmosphere for the first-stage sintering, cooling, and second-stage sintering processes is nitrogen or argon.

3. The method for preparing lithium manganese iron phosphate cathode material according to claim 1, characterized in that, The second mixing process involves adding the lithium manganese phosphate material, the iron source, the second phosphorus source, the second lithium source, the dopant M, and the carbon source to water to prepare a second mixture. The total mass fraction of the lithium manganese phosphate material, the iron source, the second phosphorus source, the second lithium source, the dopant M, and the carbon source in the second mixture is 20%-60%. The second mixture is then subjected to a second sand milling process to control the particle size D50 of the solids in the second mixture to be 0.35~0.60 µm.

4. The method for preparing lithium manganese iron phosphate cathode material according to claim 1, characterized in that, The dopant M includes at least one of the compounds of magnesium, titanium, vanadium, niobium, chromium and cerium; And / or, the carbon source includes at least one of glucose, sucrose, polyvinyl alcohol, polyacrylamide, polyvinylpyrrolidone, chitosan, melamine, fructose, cyclodextrin, and polyethylene glycol.

5. The method for preparing lithium manganese iron phosphate cathode material according to claim 1, characterized in that, The molar ratio of lithium in the second lithium source, iron in the iron source, and phosphorus in the second phosphorus source is (1-1.05):1:(1-1.03). And / or, the molar ratio of the metal element in the dopant M to the iron element in the iron source is (0.005-0.05):1; And / or, the molar ratio of iron in the iron source to manganese in the lithium manganese phosphate material is (1-5):(5-9); And / or, the mass of the carbon source is 10%-20% of the theoretical yield mass of carbon-free lithium manganese iron phosphate.

6. The method for preparing lithium manganese iron phosphate cathode material according to claim 1, characterized in that, The preparation method of the lithium manganese phosphate material includes: mixing a manganese source, a first phosphorus source, a first lithium source and a dopant N in a first mixing process, freeze-drying to obtain a dried material; and sintering the dried material to obtain the lithium manganese phosphate material.

7. The method for preparing lithium manganese iron phosphate cathode material according to claim 6, characterized in that, The manganese source includes at least one of manganese oxides, manganese hydroxides, soluble manganese salts, manganese carbonate, and manganese phosphate. And / or, the first phosphorus source or the second phosphorus source includes at least one of phosphoric acid, ammonium dihydrogen phosphate, diammonium hydrogen phosphate and phosphorus pentoxide; And / or, the first lithium source or the second lithium source includes at least one of lithium carbonate, lithium hydroxide, lithium phosphate, lithium oxalate, lithium acetate and lithium nitrate; And / or, the dopant N includes at least one compound of magnesium, titanium, nickel, aluminum, vanadium, niobium, cobalt, chromium and cerium.

8. The method for preparing lithium manganese iron phosphate cathode material according to claim 6, characterized in that, The first mixing process includes adding the manganese source, the first phosphorus source, the first lithium source, and the dopant N to water to prepare a first mixed solution, wherein the total mass fraction of the manganese source, the first phosphorus source, the first lithium source, and the dopant N in the first mixed solution is 20%-60%; and the first mixed solution is subjected to a first sand milling to control the particle size D50 of the solids in the first mixed solution to be 0.2~0.4 µm. And / or, the molar ratio of manganese in the manganese source, lithium in the first lithium source, and phosphorus in the first phosphorus source is 1:(1-1.05):(1-1.03). And / or, the molar ratio of the metal element in the dopant N to the manganese element in the manganese source is (0.005-0.05):

1.

9. The method for preparing lithium manganese iron phosphate cathode material according to claim 6, characterized in that, The sintering atmosphere includes at least one of nitrogen, argon, 95% nitrogen / 5% hydrogen, and 95% argon / 5% hydrogen. And / or, the sintering temperature is 500-650℃ and the sintering time is 2-8h.

10. The application of the lithium manganese iron phosphate cathode material prepared by the method described in any one of claims 1-9 in lithium-ion batteries.

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